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Westbury The Tube at GEO Function of diodes


 
4/11/1999 3:07 PM
C. Hütter
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Westbury The Tube at GEO Function of diodes
I've just seen the schematics of the Westbury The Tube at R.G's website. Obviously a tube overdrive pedal(?).  
What is the function of the diodes in the feedback loop?  
 
Carsten
 
4/11/1999 4:24 PM
marrk
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The circuit looks like a fuzz pedal with the tube in place of the transistors. The diodes appear to be there for distortion. They are not directly to ground so maybe they clip softer than a fuzz. Mark
 
4/11/1999 8:19 PM
R.G.
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Wow, I just put that up a couple of hours ago.  
 
It's a tracing of a pedal effect by that name, tube overdrive pedal as you note.  
 
The diodes? That's not real clear to me right now. I *think* that they serve the same function as the feedback clipping diodes in the Big Muff Pi, limiting the output, but with the sharpness of clipping limited by how hard the tube can drive them.
 
4/12/1999 10:20 AM
John Greene
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The diodes? That's not real clear to me right now. I *think* that they serve the same function as the feedback clipping diodes in the Big Muff Pi, limiting the output, but with the sharpness of clipping limited by how hard the tube can drive them.  
 
Correct me if I'm wrong (it's highly likely this early on a Monday morning) but I see the diodes providing what would be positive feedback to the first stage. It would be in phase with the input signal at the grid. However, it's being injected into the Cathode which is bypassed by a 25uF capacitor. This to me would kind of 'negate' the frequency effect that bypassing the Cathode has (less gain at lower frequencies). Since the diodes are in series, they don't come into play until the signal is over .6V or so.  
 
The electrolytic connecting the diodes to the output appears to be connected in the wrong polarity as well.  
 
If time permits, I'll try simulating it today.  
 
--johng
 
4/12/1999 10:43 AM
GFR
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I think the feedback would not be positive because the loop is connected to the cathode of the first stage. But since the cathode is bypassed with a large cap, for all practical purposes it is AC ground. Except for very low frequencies it's like the diodes were connected to ground. For very low frequencies the open loop gain is low because of the small plate coupling caps.  
 
I think it's just two triode stages with a diode clipper to ground, only the "ground" is at an unnusual circuit point. Maybe this connection has some advantages for hum rejection?  
 
That cap (10uF/50V) looks like it's at the wrong polarity for me too.
 
4/12/1999 11:03 AM
John Greene
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I think the feedback would not be positive because the loop is connected to the cathode of the first stage.  
 
Hmmm, I think the feedback would be positive for the same reason. If the feedback came from the first plate, it would be negative because the tube inverts the signal. Since it's coming from the second stage, the signal is inverted twice making it positive, no?  
 
Anyway, from what I have been able to see through simulation, it's just like you said. It's basically AC ground. I think the effect at low frequencies will never make it to the 25uF cap because of the 10uF cap connecting the diodes to the output.  
 
There was a small difference in the frequency response of the circuit with and without the diodes but I don't think it's enough to make a difference.  
 
I wouldn't be surprised if it was done as an attempt to protect the design. If another company copied the design exactly, they could prove it by showing the diodes connected the way they are have no effect on the signal. The only reason someone else would put it in there is because they copied the circuit.  
 
--johng
 
4/12/1999 12:20 PM
Roger
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quote:
"If the feedback came from the first plate, it would be negative because the tube inverts the signal. Since it's coming from the second stage, the signal is inverted twice making it positive, no? "
 
 
It would be positive feedback if it were connected to the grid of the first tube, but connected to the cathode, it is negative feedback... that is, if all of the signal wasn't going through the bypass cap to ground.  
 
I agree with GFR that because of the bypass cap on the cathode resistor of the first stage, the diodes might as well be connected to ground. If you take off that bypass cap then that's another story.  
 
Are we sure the 10uF cap coupling the diodes is after the output cap and not on the plate of triode 2?  
 
Roger K.
 
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